The present assignee has developed a printable LED light sheet where microscopic inorganic LED chips, having a top electrode and a bottom electrode, are printed as an ink on a conductive layer on a thin substrate. Such LEDs are called vertical LEDs. After the ink is cured, the bottom electrodes of the LEDs make electrical contact to the conductive layer. A dielectric layer is then deposited between the LEDs, and another conductive layer is printed to make electrical contact to the top electrodes of the LEDs to connect the LEDs in parallel. A suitable voltage is applied to the two conductive layers to illuminate the LEDs. To allow light to escape, one or both of the conductive layers is transparent. Indium tin oxide (ITO) or sintered silver nano-wires are preferred for the transparent conductive layer. With nano-wires, after the nano-wire ink is printed and cured, the nano-wires form a sintered mesh with spaces between the nano-wires to allow the light to pass.
One desired application of the light sheet technology is for large area lamps, such a 2×4 foot lamp to replace conventional fluorescent troffers. Other large area applications are envisioned.
The practical sheet resistance of the printed ITO layer is typically 50-100 Ohms/square and, for silver nano-wires, it is typically about 5-10 Ohm/square. For large light sheets, the currents conducted by the conductive layers are large so there will be significant voltage drops across the light sheet resulting in brightness non-uniformity. Thicker layers of the transparent conductor can lower the resistance, but this limits transparency, makes it more difficult to fabricate, reduces flexibility, and adds cost. As a result, the transparent conductive layer can only be optimized for a relatively small LED light sheet, limiting the practicality of using the technology for large area light sheets.
What is needed is a technique for forming a larger area LED light sheet of any size that does not suffer from the above-described problems with the transparent conductive layer. Further, the technique should allow the lamp to be formed using a roll-to-roll process.